The invention relates to a knife assembly of the knife carriage for a veneer lathe. The knife assembly comprises heads having mounted therebetween a knife beam and nose bar beam, these beams respectively supporting the peeling knife of the lathe and the nose bar backing the outer periphery of the log adjacent the point where the veneer will be separated during peeling. The combination of the knife and nose bar beams with the knife and nose supported thereby, and the heads form the knife assembly of the lathe.
Conventionally, the knife assembly is arranged to be movable, together a proper carriage on which the assembly is mounted, in a veneer lathe as an entity along a track in relation to the log being peeled in order to provide a feed movement of the knife assembly at a rate synchronized to the progress of peeling. Additionally, the position of the knife assembly, and thus also the peeling knife and the nose bar respectively is arranged controllable. Such a control facility is implemented by way of making the angular position of the knife assembly variable about an axis of rotation that is substantially parallel to the longitudinal axis of peeling knife and the nose bar. Generally, the term xe2x80x9csubstantially parallelxe2x80x9d is in the art understood to mean perfect parallelism, but small variations therefrom are acceptable depending on the actual peeling situation. This type of angular position control of the knife assembly makes it possible to implement a correct alignment of the peeling knife in regard to the log during peeling. In this control arrangement, the peeling knife and the nose bar rotate as an entity in the knife assembly, whereby the mutual angular position between these knife assembly elements does not change.
It is further known in the art that the distance of the peeling knife from the nose bar, so-called knife gap; is adapted adjustable. Herein, the knife gap is set narrower than the nominal thickness of veneer to be peeled by a value called the degree of log compression. The degree of log compression defines the pressure force with which the nose bar running on the periphery of the log compresses the log surface radially inward immediately before the periphery of the log meets the cutting action of the peeling knife. The degree of log compression may be varied according to the progress of peeling through the different parts of the log. Obviously, the softer portion of the log may be peeled using a different degree of log compression than that required for the harder portion of the log. While the resilience of the log portion being peeled is dependent on the wood species, a general rule is that sapwood is softer than heartwood. However, veneer may also be peeled from wood species in which the situation is reversed. Hence, it is necessary to provide means for changing the degree of log compression according to the wood species or state of the log being peeled.
As known in the art, the nose bar may be either a so-called solid nose bar, whose backing surface glides on the periphery of the log being peeled, or a so-called roller nose bar that may be, e.g., a small-diameter roller arranged to roll along the log periphery. By varying the counterforce imposed by the nose bar, the quality of veneer obtained from peeling may be affected substantially.
In peeling using a solid nose bar, it has been found that a significant factor as to the quality of the veneer produced by the lathe is the face angle of the nose bar, which in the art is defined as the inclination of the beveled face of the nose bar from the vertical plane at the zero position of the lathe, i.e. at the position where the tip of the peeling knife in the knife assembly is exactly at the height of a horizontal plane aligned at the center axis of the lathe spindle, while the nose bar is respectively set into its operating position. It is further known that the bevel angle of the nose bar must be selected according to the wood species being peeled, whereby a smaller bevel angle is needed for softer wood species. Herein, the lathe is set for the wood species to be peeled by mounting thereon a nose bar having a bevel angle experimentally optimized for the intended peeling operation. In the art are also known arrangements that permit optimized setting of the angular position of the nose bar in the nose bar beam prior to starting peeling.
According to the present invention, the performance of a veneer lathe is improved by virtue of a knife assembly comprising a peeling knife and a solid nose bar with a continuous adjustment facility for setting the angular position of the nose bar relative to the peeling knife thus allowing the angle between the nose bar and the peeling knife to be changed during peeling. In the present construction, the angular position of the nose bar can be set and controlled independently from the angular position of the peeling knife.